The band structure of RMG with \(N=16\)  layers for a single spin-valley flavor is shown in Fig. REF (a,b) for non-interacting and interacting electrons, respectively, the latter as determined by numerical solving the mean-field theory (REF ,) for \(2N\)  bands, Eq. (REF ).
The orbitals on the surface sites, \(A_1\)  and \(B_N\) , contribute to a pair of low-energy bands that are flat for \(k k_c\)  where \(k_c = \gamma _1 / \hbar v\) . The wave vector \(k_c\)  corresponds to the point of the phase transition between non-trivial and trivial topological phases in the SSH model [1]}, [2]}, [3]}, [4]}. For the interacting ground state, the band structure displays flavor degeneracy, Fig. REF (b).
The band gap, \(E_{\mathrm {g}}\) , is at finite wave vector \(k\)  between the surface state bands, this is the difference in energy between the conduction band minima and the valence band maxima. We also consider the separation of the surface state bands at \(k = 0\)  which we refer to as the order parameter \(\Delta \) . In principle, it is possible to have non-zero \(\Delta \)  even when \(E_{\mathrm {g}} = 0\)  and, generally, \(\Delta \ge E_{\mathrm {g}}\) .
